This invention relates to a method for the manufacture of a water-resistant paper container and more particularly, to a method for economically manufacturing a water-resistant paper container having a thermoplastic film layer on the inner wall and/or the outer wall surfaces of the container by means of ultrasonic wave welding.
Hitherto, there has been a great deal of demand for a paper container which can store a liquid or high water-content product such as milk, yogurt or the like for a long period of time without the container contents leaking and/or soaking into the paper and which can be simply and economically manufactured. Said leakage and soaking impair the strength or appearance of the container. However, to date, no container which can satisfactorily comply with such demands of users has been obtained. It has been known that a highly water-resistant paper container can be obtained if the interior and/or exterior wall surfaces of the paper container are covered with a thermoplastic film such as polyethylene film, polypropylene film or the like. Especially, a paper container lined on the interior wall surface with a thermoplastic film and coated on the exterior wall surface with the same or similar film has extremely high water-resistance for the reason that there are no uncoated portions on the interior wall surfaces of the container, and therefore, the contents of the container can not penetrate into the paper or leak outside and the plastic film coated on the exterior wall surfaces of the container protects the outer surface of the container against moisture attack. Thus, this type of paper container is most preferable for storing liquid-like products. In fact, such a container is free of such problems as penetration of the contents of the container into the paper, leakage of the contents from the container and impaired strength of the container, even after it has held the contents for a very long period of time. Also, since the exterior wall surface of the container is coated with a thermoplastic film such as polyethylene film, atmospheric moisture can not penetrate even when the container is placed in a highly humid environment, for example, in a refrigerator, and therefore, the strength of the container is not affected by the moisture. However, the manufacture of this type of container has encountered some inherent technical difficulties.
For example, the conventional method for manufacturing such a container coated on the exterior wall surface with a thermoplastic film uses a suitable adhesive for joining the container body portion and the container bottom panel together. More particularly, in the adhesive bonding of the container body portion and the container bottom panel, the adhesive is applied in a strip form to the uncoated paper surface of the container body blank, which is to be the inner wall of the container, slightly above the lower end of the blank; the peripheral flange of the bottom panel formed by downwardly bending the outer peripheral edge of the bottom panel at right angles to the plane of the bottom panel is positioned on the adhesive layer coated on the paper surface of the body blank; the lower end of the container body blank is inwardly and upwardly bent around the peripheral flange of the bottom panel so as to encompass said peripheral flange of the bottom panel with the inwardly and upwardly bent flange of the container body to thereby form a bottom seam; and then the so-formed bottom seam portion is mechanically clamped on both sides of the seam to firmly join the container body member and the bottom panel together. Therefore, the interior wall surface of the so-formed container member is lined with a thermoplastic film by a known method, for example, the plug-assist forming method or the plug-assist blow forming method. In each method, a preheated and softened sheet of the thermoplastic film is positioned across the opening of the container so as to hermetically seal said opening, said sheet is then pressed down into the interior of the container by the plug, and then after the plug has reached its dead point, vacuum forming means or pressure forming means is applied to the container to thereby manufacture a container lined with the thermoplastic film. However, in order to achieve the firm contact of the film with the whole inner wall surfaces of the container in each method, the air entrapped in the enclosed space defined by the lining film and the inner wall surface of the container member need be removed from the space. Heretofore, the entrapped air has been removed from the container through the bottom seam portion of the container. However, where an adhesive is used for bonding the peripheral flange of the bottom panel and the container body wall, the two components are adhesively joined together at the whole area of the interface between the components so that there are very few voids in the bottom seam portion through which the entrapped air can be evacuated. Accordingly, air exhausting will require a rather long time which makes it difficult to manufacture a water-resistant paper container in a commercially acceptable period.
Furthermore, the conventional method referred to hereinabove also employs an adhesive in the formation of the container body member. That is, an adhesive is applied in a strip form to one cut end of the container body blank, which is coated on the exterior side with the thermoplastic film, in the longitudinal direction of the end and then the blank is rolled into a cylindrical shape so as to bring the opposite cut ends of the blank into slightly overlapping relationship to thereby form an adhesively bonded side seam which extends in the longitudinal direction along the container body wall. However, in this method, the paper surfaces are not bonded together by the adhesive, but the paper face and the film face are bonded together by the adhesive, so that, the bond strength between the film face of one end of the blank and the paper face of another end of the blank is not so high and, therefore, the side seam formed by bonding the film face and the paper face with the adhesive is weak and the two faces tend to easily separate from each other.
Moreover, in the conventional manufacturing procedure of the paper container, before or after the step of lining the inner wall surfaces of the container member with the thermoplastic film, the outer upper periphery and its vicinity of the container side wall is heated by suitable means to slightly soften the same, and then the so-softened upper periphery of the side wall is outwardly curled or bent so as to form a mouth edge or rim of the container, preferably with a horizontal upper end. The purpose of providing the curled mouth edge with the horizontal upper end is to facilitate hermetical sealing of the container filled with contents such as liquid foods or the like by joining at said mouth edge of the container with a lid member formed of a sheet material of a thermoplastic film such as polyethylene film or of a composite sheet of such film and a metal foil such as an aluminum foil. However, the conventional method for forming the mouth edge of the container merely consists of outwardly curling and mechanically clamping the curled flanges and does not include any step or means for bonding the curled flanges together. As a result, as time elapses after forming the mouth edge of the container, the curled flanges of the mouth edge are apt to gradually turn back to the previous uncurled state, thus giving the container a deformed mouth edge. However, the deformed mouth edge is a cause of decreased production efficiency prevents smooth hermetic sealing, for example by heat, of the container with the lid member after the container is filled with its contents.